1. Field of the Invention
The present invention relates to a method and apparatus which reduce the amount of computation in audio post-processing, and more particularly, to a method and apparatus which recover a high frequency component in MPEG-1 layer 3 audio using stereo mode information detected in a decoder, by selecting a mode depending on a circumstance after dividing stereo modes into two modes. In a first mode, the high frequency component of only one channel is recovered and the recovered high frequency component is used to recover the high frequency component of the other channel when the difference between the two channel signals is small. In a second mode, the high frequency component in only one of every two frames is recovered alternately in the left channel and the right channel and the high frequency component of each of the skipped frames is interpolated based on the high frequency components of the previous frame and the next frame when the difference between the two channel signals is big.
2. Description of the Related Art
In an MPEG-1 layer 3 audio coding method, in order to compress data more efficiently a psychoacoustic model is used in which less bits are allocated to a high frequency component that humans cannot hear. In this case, although the compression ratio improves, high frequency components are lost. Because of the loss of the high frequency components the tone changes and the clarity of sound degrades such that depressed or dull sounds are output. Accordingly, in order to reproduce the tone of the original sound and to improve clarity, a post-processing sound quality improving method for recovering lost high frequency components is needed.
However, despite their high sound quality conventional post processing sound quality improving methods need an excessive amount of computation to perform the post processing methods. A sub-band replication (SBR) method, which is a widely-known post-processing sound quality improving method, requires about double the number of computation, performed in an MPEG-1 layer 3 decoder.
A post processing method by which a sound signal decoded in a time domain is provided in order to achieve a high frequency resolution from a frequency spectrum having a combination of a non-high frequency component and noise is disclosed in Korea Patent Laid-Open Gazette No. 2000-75936.
For a high frequency resolution the method comprises converting a time domain signal that is fast Fourier transform (FFT)-decoded into a frequency domain signal; analyzing an energy distribution in order to detect a disturbance frequency component in the frequency domain signal; detecting the amount of suppression amount on the disturbance frequency component; controlling post filtering; and reverse-converting the data converted by the post filtering.
However, the method for post processing is performed in a voice decoder to improve perceptual quality of a decoded voice when voice and audio signals are decoded. Although the purposes for improving the resolution of a high frequency component are similar, the steps of the conventional post processing are different from the present invention because the conventional post processing method comprises analyzing a frequency spectrum, and using resolution filtering high frequency. In addition, the effect of reducing the amount of computation in performing the post processing method cannot be achieved in the prior art method.